Spin-wave excitations in multiferroic heterostructures and CoFeB/YIG bilayers

Sampo Hämäläinen

Research output: ThesisDoctoral ThesisCollection of Articles


Spin waves are collective excitations in magnetic materials that can exhibit long coherence lengths, high group velocities, and wavelengths down to the nanometer scale. The properties of spin waves could be exploited in low-power wave-like computing and in other devices such as microwave filters and data storage elements. In contrast to the motion of electrons, spin waves propagate via transmission of angular momentum. Power dissipation by Joule heating can thus be avoided.  In this thesis, I present results on spin-wave excitation and confinement in strain-coupled multiferroic heterostructures and YIG/CoFeB bilayers. The multiferroic heterostructures consist of a ferroelectric BaTiO3 substrate with a regular pattern of ferroelastic stripe domains and a ferromagnetic film. Strain coupling at the interface imprints an anisotropy pattern in the ferromagnetic film via inverse magnetostriction. As a result, the domain pattern of the BaTiO3 substrate is fully transferred to the ferromagnetic film and magnetic domain walls are firmly pinned by the induced anisotropy boundaries. The pinned magnetic domain walls can be used to excite short-wavelength spin waves using an ac electric current. In this case, the spin-polarized current drives the domain wall into oscillatory motion, causing spin-wave emission at the same frequency as the actuation signal. If the same multiferroic system is saturated by an external magnetic field and excited by a uniform microwave field, standing spin waves form within the domains and propagating spin wave are emitted from the anisotropy boundaries. Here, rotations of the anisotropy axis with respect to the direction of magnetization create a regular modulation of the effective field. The launching of spin waves from a single anisotropy boundary is explained by coupling between two forced magnetization precessions in neighboring stripe domains. A similar mechanism is also responsible for the excitation of perpendicular standing spin waves in exchange-coupled YIG/CoFeB bilayers. In this system, forced precession in YIG and CoFeB generate a dynamic exchange torque at the interface. Lastly, multiferroic heterostructures are used to study spin wave transmission through narrow and wide 90° domain walls. Deterministic switching between these wall types is realized by realigning the magnetization from a head-to-tail (narrow wall) to a head-to-head (broad wall) configuration. Brillouin light scattering experiments and micromagnetic simulations demonstrate that this reversible switching effect toggles the transmission of spin waves from nearly 0% to 100%. Reconfigurable modulations of spin-wave transmission could be used in logic devices or non-volatile memory cells.
Translated title of the contributionSpin-aaltoviritykset moniferroisissa heterorakenteissa ja CoFeB/YIG kaksikerrosrakenteissa
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • van Dijken, Sebastiaan, Supervising Professor
Print ISBNs978-952-60-8204-2
Electronic ISBNs978-952-60-8205-9
Publication statusPublished - 2018
MoE publication typeG5 Doctoral dissertation (article)


  • spin waves
  • multiferroic heterostructures
  • magnonics


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